1. Field of the Invention
This subject matter relates generally to semiconductor devices and techniques for their manufacture, and more particularly, to semiconductor devices and manufacturing techniques which include electrolessly deposited metallization.
2. Discussion of the Related Art
Typically, in the manufacture of high density semiconductor devices, multiple layers of conductive material connected by vias or plugs are provided above a substrate. Such multiple layers, typically of metal, are employed in order to accommodate high device densities.
Recently, copper has received significant attention for use as via material in VLSI (very large-scale integration) and ULSI (ultra large-scale integration) applications. In particular, electroless copper deposition has been indicated as a desirable process applicable to 100 nm-damascene metallization because of its high conformality, low deposition temperature, high via/trench filling capability and low processing cost. However, electroless deposition of copper requires the activation of a surface on which the electroless deposition takes place. Furthermore, since copper conductors require isolation from adjacent material, the electroless deposition of copper must generally be performed on a barrier layer, i.e., the activation process must be performed on the barrier layer. An example of such a process is shown and described in U.S. Pat. No. 5,891,513, entitled xe2x80x9cElectroless CU Deposition On a Barrier Layer by CU Contact Displacement for ULSI Applicationsxe2x80x9d, Valery M. Dubin et al., issued Apr. 6, 1999.
The above-cited patent discloses a process wherein a surface of a barrier layer is activated for electroless deposition of copper by bringing the barrier layer surface into contact with an aqueous copper contact displacement solution. Such contact forms a seed layer of copper on the barrier layer surface. Copper is then electrolessly deposited on the activated surface. In that patent, the seed layer includes only the same metal as that which is electrolessly deposited, i.e., in that situation copper. The inclusion of other metals in the seed layer (such as tin or palladium) is purposely avoided.
A significant problem faced with metal conductors is that of electromigration. With current flow through a metal conductor, over time, the atoms of the metal may migrate to cause a break in the metal. Although copper is relatively resistant to such electromigration, as device dimensions continue to decrease, it will be understood that an increase in a metal""s ability to withstand such electromigration is highly desirable. Ideally, this increased resistance to electromigration should be obtainable in the environment of electrolessly deposited copper described above.
Therefore, what is needed is a process which provides proper activation of a barrier layer for subsequent electroless deposition of copper thereon, meanwhile providing that the electrolessly deposited copper is highly resistant to electromigration.
In the present method, a surface of a diffusion barrier layer has a seed layer formed thereon for electroless deposition of copper. The seed layer is formed by subjecting the surface of the diffusion barrier layer to an aqueous contact solution which includes copper, tin and palladium, and hydrochloric and hydrofluoric acids. The hydrofluoric acid serves to remove native oxide from the diffusion barrier layer prior to formation of such seed layer. The proportions of copper, tin and palladium compounds in the solution are chosen so that the seed layer formed during this process promotes electroless deposition of copper thereon, meanwhile with the tin and palladium in the seed layer causing the free migrated copper at the copper surface to be reduced, due to tin and palladium binding with the copper. Preferably, the seed layer contains copper, tin and palladium substantially in the following atomic % percent ratios:
Cu:Sn:Pd: 12:1:0.01.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.